This invention relates generally to magnetic tape transports, sometimes referred to as tape units, tape drives, tape decks, etc., for use with a replaceable tape cassette or cartridge, and particularly to those used as a subsystem to enable a host system to obtain access to data on magnetic tape. More particularly, the invention concerns a method of, and means for, automatically taking up a slack that may develop in the tape it, during its travel in a cassette tape transport under the control of a tape speed control servomechanism.
Cassette tape transports, particularly to those operating in streaming mode, as contrasted with start/stop mode, have come to find extensive use as peripherals of computer systems. U.S. Pat. No. 4,163,532, filed by Sakai and assigned to the assignee of the instant application, discloses one such streaming cartridge tape transport, or streamer, which bears particular pertinence to the present invention. This prior art streamer comprises a pair of reel motors for bidirectionally driving the pair of reels of an associated tape cartridge under the direction of a motor control circuit forming a part of a tape speed control servo. Also included in the tape speed control servo is a tape speed sensor comprising a roll for frictional engagement with the tape, and an encoder for generating pulses indicative of the tape speed. The motor control circuit drives the tape reels at constant speed in response to the speed sensor output pulses.
The tape must of course travel under proper tension between the reels in order to enable the head to correctly write or read data on the tape. To this end the prior art streamer additionally comprises a tension control circuit which affords constant tape tension in the face of varying tape diameters on both reels.
The streaming tape transport of the noted prior art construction has proved to have a weakness in connection with the tape speed control servo. The load on the tape transport mechanism will change abruptly, and the speed sensor will fail to operate normally, when vibrations or shocks are exerted on the device during tape travel at constant speed. The spacings of the tape speed sensor output pulses will unduly extend in such cases, falsely indicating a drop in tape speed.
Heretofore, the motor control circuit has responded to such unduly long pulse spacings by accelerating one of the reel motors. Since such sudden acceleration inevitably involves an overshoot, the motor control circuit has then had to decelerate the motor, resulting in hunting in some cases. The hunting of the tape speed control servo has led in turn to the development of a slack in the tape. The tape on slackening has been easy to be physically destroyed by jamming.